Knowledge of the in situ stresses and material properties of the earth's crust and surficial deposits is essential for rational analysis of geological and man-made structures. Determination of in situ stresses and material properties based on laboratory testing of core samples may result in unacceptably large errors, due to property deterioration of samples which have been removed from their natural location. In situ measurement of stresses and properties can eliminate these errors, yet suitable instrumentation for gathering in situ data has not been available in the prior art.
Important applications of in situ stress and property measurement occur in geotechnical engineering. The load on underground structures cannot be calculated without first determining the preexisting stress field in the underground media. Underground openings are being used increasingly for transportation systems and storage facilities; this increased use emphasizes the need for rational design procedures based on an accurate assissment of in situ stress and material conditions. The in situ stress conditions in underground media are also a major factor in the design of foundations for major structures. Among the more important applications are underground cavities for the storage of liquids, gasses, and solids, including such materials as oil, gas, compressed air, petrochemicals, and nuclear wastes. Also, the safe design of mine openings, both for conventional and solutioned mines, requires an accurate knowledge of existing stress states in the surrounding underground media. This is also true in the design of tunnels, galleries for hydropower generators, and foundations for large structures such as dams, high-rise buildings, and off-shore platforms. An important application of in situ stress and property measurement maybe found in future prediction of earthquakes. The accurate determination of in situ stresses and their time-dependent change is necessary to improve our understanding of regional geological stress fields and the dynamics of tectonic plate systems. Quantitative evaluation of local stresses and material properties along fault planes is also of major importance in the development of procedures for predicting earthquakes. The prior art reveals a paucity of devices which can achieve such quantitative evaluation over long periods of time while disposed in a borehole in seismically active geological formations.
The prior art instrumentation for the determination of in situ stress fields and material properties are broadly classified into three categories: pressure meters, hydraulic fracturing devices, and overcoring methods.
Hydraulic fracturing and overcoring methods are limited to applications in competent rock. Neither method gathers any data related to material properties. Overcoring techniques are difficult, time-consuming, and rather expensive to perform. Furthermore, none of these techniques are applicable in ground media not ideally uniform and elastic. Also, the depth at which measurements can be made is restricted by the need to overcore and perform complex manipulations from outside the borehole. As is the case for the overcoring method, the interpretation of results from hydraulic fracturing methods relies on the assumption of linear elasticity and homogeneity. It is also assumed that one of the principle stresses in the ground media is parallel to the borehole axis. Therefore, the hydrofracturing method is not applicable in those cases in which the stress in the direction parallel to the borehole is substantially smaller than the stress in the other principal directions. Furthermore, hydraulic fracturing techniques cannot be used in ground media which is already fractured or which is highly permeable.
A variety of simple pressure meters has been developed to measure the elastic modulae of rocks through analysis of volume changes in a pressurized cell introduced into a borehole. More advanced forms of this type of instrument have been developed for the determination of lateral earth pressure in soils at rest, as well as for the determination of some generalized material properties and bore pressures. These instruments cannot discriminate nor detect the directional components of stress fields and their depth capabilities are limited by the available soil mechanics boring equipment.
None of the prior art techniques are applicable to in homogenious, anisotropic ground, as none yields information regarding the visco-elastic and visco-plastic time dependent rheological material properties of earth materials.